The putting green is responsible for half of the strokes in a round of par golf. The spectacle of professional golfers missing four-foot putts is well-known. The reason is complex and elusive. The light forces involved result in the need for delicate muscle control reaching almost to the level of surgical skill. To make analysis even more difficult, there are a few optical illusions to contend with, and some complex projection geometry. For reliable and consistent accuracy, the club face must be square to the proposed path of the ball at the instant of impact, and also either (a) the path of the club head at the point of impact must be tangential to the ball path, or (b) the movement of the point of impact must be in a vertical plane containing the path of ball movement. If a putting stroke is based upon (a), the point of impact must be precisely located along the line of the ball path, or slight lateral movement of the head with respect to the ball path occurs. This adds another variable. If a stroke is based upon (b), there must be a high order of skill in converting what tends to be a swinging from an indeterminate axis into a movement that does not allow the club head to shift to either side of the ball path. The optical illusions center in the fact that the player's eyes are rarely directly above the ball, and are thus usually out of the vertical plane containing the ball path. From this viewpoint, curved lines of movement of the club head can appear straight, and vice versa.
Golfers have long been groping for assistance in developing putting skills. Elaborate training devices have been devised and developed, some involving complex electronic equipment giving all sorts of readout information, all at very considerable cost and handling inconvenience. Another type of device provides confinement rails for forcing the user to cause the club head to follow a particular path of swinging movement. A common and very simple system involves the stretching of a line between pegs in the ground, and stroking the putter over it. When something goes haywire in a golfer's putting in the middle of a round, something is needed right then to guide him in finding out where his coordination has gone bad, and correcting it. That something should be (1) easily handled, (2) occupy zero to minimum space in the golfing bag, and (3) be unlikely to be mislaid.